Geothermal turbine

ABSTRACT

A geothermal turbine comprises a turbine casing, a turbine rotor shaft, a plurality of nozzle diaphragm outer rings fixed to the turbine casing, a plurality of nozzle diaphragm inner rings located radially inside the nozzle diaphragm outer rings, a plurality of nozzles placed between the nozzle diaphragm outer rings and the nozzle diaphragm inner rings, which form a steam passage, a plurality of rotor blades mounted on the turbine rotor shaft, which face the nozzles and form a blade cascade in a circumferential direction, a shroud arranged on tip of the rotor blade, an overhang attached to the nozzle diaphragm outer ring, which extends downstream of the nozzles and has an inner surface facing radially outside the shroud, and a plurality of sealing fins which protrude radially outward from the shroud and face the overhang.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.10/351,613, filed Jan. 27, 2003 now U.S. Pat No. 6,860,718, the entirecontents of which are incorporated herein by reference.

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-18901 filed on Jan. 28,2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns a geothermal turbine which prevents ageddeterioration and is capable of longer operating life, and a geothermalturbine having turbine components, such as blades, nozzle diaphragms andturbine rotors, are improved in terms of corrosion resistance or erosionresistance.

2. Description of the Related Art

Geothermal steam heated by underground magma is made available bydrilling a well, and rotation power of a geothermal turbine is generatedby directly introducing this steam as a working fluid into the turbine.Energy of the geothermal steam is changed into kinetic energy in aprocess of expanding the geothermal steam through turbine stage composedof nozzles and rotor blades, and this kinetic geothermal steam activatesthe rotor blades and thus generates power to rotate a turbine rotorshaft on which the rotor blades are mounted.

Since the geothermal steam is generated and heated by geothermal sourcessuch as magma, the steam contains corrosive components such as hydrogensulfide or sodium, scale components such as silicon dioxide or calcium,and solid particles such as sand, mud or ferrous oxide. Since thegeothermal steam is fully or almost in a saturated state, the geothermalturbine is forced to operate under severe operating conditions due tohigh wetness of the steam inside the turbine. Moreover, the variety andconsistency of the chemical composition of the geothermal steam, thesize and the amount of solids carried into the geothermal turbine, andthe steam condition of the geothermal steam, etc. differ by geothermalareas and wells in which the geothermal turbine is installed, and thesefactors even vary considerably over time even if the wells are same.These differences and aging make the design of the geothermal turbinemore complicated.

Since the geothermal steam contains corrosive gas such as hydrogensulfide, one concerns the factor shortening of the life of thegeothermal turbine regarding the rotor blades, the nozzles and the rotorshaft, such as age deterioration and damage resulting from stresscorrosion cracking or corrosion fatigue, in addition to corrosion of thematerials. For example, generally, damage by the corrosion fatigueoccurs as a result of three overlapping factors, namely, theenvironment, material and stress. It is difficult for the turbinecomponents to avoid the overlapping of these three factors completely,because the rotor blades and the rotor shaft rapidly rotate, andconsequently high stress occurs on the turbine components. Thus,subsequently, the material applicable to the geothermal turbine and thelength used for geothermal turbine may be limited in a practical aspect,and the output of the geothermal turbine is restricted within a certainupper limit.

As a realistic measure of the geothermal turbine against corrosiveenvironment peculiar to geothermal sources, suitable materials areselected in accordance with the results of evaluation examination ofvarious candidate materials that are preliminarily held in an atmosphereof the geothermal steam, concurrently with setting the stresses on eachpart of the geothermal turbine at a level lower than that of an ordinaryturbine for thermal power plant, etc. In other words, efforts to reducethe impact from the environment have been performed.

On the other hand, high wetness of the geothermal steam and solidparticles carried over in the steam may give rise to the possibility ofcausing drain (liquid) erosion or particle erosion of the steam passageor sealing portion of each part of the turbine. The wet steam and solidparticles, together with the corrosive gas contained in the geothermalsteam, may cause synergy of erosion and corrosion and become a factoraccelerating damage to the turbine. For this reason, as a measure forpreventing the damage, a drain catcher is arranged at the exit of eachturbine stage for discharging the water droplets and solid particlesoutside of the steam passage, and an erosion shielding is attached tothe tip of the last stage blade.

In an actual geothermal turbine plant, in spite of such measures forpreventing damage to the turbine against aging, severe degradation ofeach component in the geothermal turbine may nevertheless be observed.Thus, improvement of corrosion resistance and erosion resistance of eachcomponent of the geothermal turbine has been an important objective.

To convert thermal energy of geothermal steam into the rotational energyin the geothermal turbine with high efficiency, firstly, it is necessaryto flow the working steam into the nozzles and the rotor blades of thegeothermal turbine. Since the turbine rotor shaft rotates inside astationary casing, a gap is required respectively between the rotatingblade tips and the stationary nozzle diaphragm outer ring, and betweenthe turbine rotor shaft and the nozzle diaphragm inner ring. It isimportant to minimize amount of leaked steam bypassing the steam passagethrough the gaps between stationary and rotating parts.

To minimize the leakage flow through the tips of the rotor blades, aconventional geothermal turbine is equipped with seal fins arranged onan inner periphery of an overhang of the nozzle diaphragm outer ringfacing radially outside the shrouds of the rotor blades. This sealingequipment consists of multiple fins and forms a ring. The fins areextended radially inward and narrows the gap between the fins and theshrouds.

A steam sealing structure for preventing leakage of the steam on therotor shaft is the same as mentioned above, that is, multiple finsarranged on an inner periphery of the nozzle diaphragm inner ring, whichis a stationary part facing radially outside a radius of the turbinerotor shaft. These seal fins extend radially inward to narrow the gapbetween the nozzle diaphragm inner ring and the turbine rotor shaft. Asfor the sealing structure of the rotor shaft, in many cases, ahigh-and-low groove is arranged on the turbine rotor shaft to constituteso-called labyrinth seal configuration for additionally enhancing theeffect of preventing steam leakage. This labyrinth-seal configuration isalso applied to the seal structure of a gland packing portion in whichthe rotor shaft penetrates the casing.

In both cases mentioned above, it is indispensable for maintaining theefficiency of the geothermal turbine to maintain the steam sealeffective without deterioration, wastage or dropout of these steamsealing structures in a geothermal environment.

Generally, for the geothermal turbine, an axial-flow turbine shown inFIG. 11 is adopted. That is, there are a plurality of stages composed ofnozzles 1 and rotor blades 2. The rotating blades of the geothermalturbine are composed of a grouped blades structure, that is, a pluralityof circumferentially-arranged blades 2 a are connected by a shroud 3 forpreventing vibration of the rotor blades 2 excited by high-speed steamdischarged from the nozzles 1. These grouped blades effectively controlsstress due to vibration generated by the steam rotating in thegeothermal turbine within an acceptable level. These grouped blades arecomposed by fitting tenons 4 each arranged on, and coupled with, each ofthe rotor blades 2, respectively, into respective holes penetrating theshroud 3, for mortising the tenon 4 into the shroud 3, and thereby theplurality of blades 2 a and the shroud 3 are connected together.

However, if a top of the tenon 4 of each blade tip 2 a of the rotorblades 2 protrudes over the shroud 3, that is, protrudes raadially outof the shroud 3, it may be eroded and/or corroded by drain of thegeothermal steam or the solid particles, and thus the life of the rotorblades 2 may become short. To prevent loss of the life of the rotorblade, as one example, a recessed tenon may be arranged so as not toprotrude the tenon 4 over the upper surface of the shroud 3, and inaddition, multiple seal fins 5 b may be arranged on an overhang 5 a of anozzle diaphragm outer ring 5 opposed to the shroud 3, for forming astructure for preventing steam leakage at the blade tip. Thus, thisstructure for preventing steam leakage at the blade tip improves erosionand corrosion resistance of the tenon.

Moreover, for turbine stages operating in a severely corrosiveenvironment such as in a geothermal turbine, fatigue strength of therotating parts such as the blades and rotor materials is significantlydecreased due to corrosion. The decrease of the fatigue strength of theblade materials, etc., directly affects the life of the blade relatingvibration.

As mentioned above, the blade vibration stresses excited by steam forcesare suppressed by the group structure. However, when the fatiguestrength of the turbine materials significantly decreases under thecorrosive environment, and thus making difficult to keep vibrationstresses below the material fatigue limit, it is hard to avoid the riskof age damage coming from corrosion fatigue. To minimize this risk, theblade width may be preliminary arranged broader for increasing itsrigidity. In this case, since the weight of the blade increases andthereby stresses of the blade fixation and rotor wheel increase, as aresult, there occurs another aspect of risk such as stress corrosioncracking on the blade fixation, etc.

When a geothermal turbine is operated at low load, the last stage bladeoperates far from its aerodynamic design point with a substantiallyreduced output and pressure drop across the stage. This will causenonsteadiness of the flow field with large back flow. The nonsteadyturbulent flow around the last stage of the geothermal turbine acts onthe blade as a strong exciting force, and thus the last stage bladeneeds a damping structure which is more effective than that of anordinary turbine stage. For this reason, in many cases, in addition tothe tenons 4 and the shroud 3 on the tip of the rotor blade 2, acoupling member called a lacing wire 6 is arranged in the middle of theblade as shown in FIG. 12. This lacing wire 6 is constituted by leadinga wire through holes penetrating the blade and brazing them to eachother, or by simply leading a wire through the holes as loose coupling.Generally, from the viewpoint of the damping effect of the blade, theloose coupling is superior to brazing.

In the thermal power turbines, a cover piece 7 may be arrangedseparately from the blades 2 a, 2 b at the blade tip as shown in FIG.13, taking advantage of the excellent damping characteristics of theloose coupling. In this figure, the tenons 7 a, 7 b are protruded fromopposite side faces of a rhombic cover piece 7, respectively, and onetenon 7 a is inserted into a tenon hole bored at one blade tip for fixedjoint, and the other tenon 7 b is inserted into another tenon hole ofthe adjacent blade for loose coupling. Thus, this structure allows asmall movement between the blades 2 a, 2 b and the cover piece 7.Sequential connection of adjacent blades 2 a, 2 b, . . . , through coverpieces loosely around the wheel constitutes continuous coupling of theblades 360 degrees and this continuous coupling provides excellentdamping effect.

However, under a severe corrosive environment of the geothermalturbines, corrosive components deposited around the wire hole arrangedor the tenon hole may easily become a trigger of stress corrosioncracking and corrosion and fatigue. Therefore, it is difficult to adoptthis loose coupling connection structure to the geothermal turbine.

In the inlet stages of the geothermal turbines where the blade lengthsare relatively short, natural frequencies of the grouped blades arechosen so as to avoid resonance with the nozzle passing frequency (NPF).NPF, which is the product of the number of nozzles and the rotationalfrequency of the shaft, is one of the excitation frequencies of thesteam discharged from the nozzle. To avoid the resonance completely,this NPF is usually set above the lower modes of the natural frequenciesof the grouped blades. The natural frequencies of such short blades arerelatively high, and consequently, NPF must be set higher, resulted in alarge number of nozzles.

Since the number of nozzles is inversely proportional to a size of thenozzles, the dimension size of the nozzle of these turbine stagesbecomes small. Arranging small nozzles for the turbine stages with shortblade heights provides larger aspect ratio, that is the blade heightdivided by the blade width, and which might have better influence onstage efficiency. On the other hand, in this case, there is thedisadvantage that resistance may be lowered against deterioration anddamages inherent to the geothermal turbine, such as damages due to solidparticle erosion, deterioration caused by corrosive components, andscale deposit.

The steam flow at the nozzle exit of geothermal turbines has a largecircumferential velocity component. As the nature of geothermalturbines, when steam includes droplets (liquid components) and solidparticles, a centrifugal force shifts them radially outward. As shown inFIG. 11, sealing fins 5 b arranged at the overhang of the nozzlediaphragm outer ring may dam the particles centrifuged aside. However,because the strong circumferential velocity dominates in this portion,the droplets and the solid particles repeatedly circulate in a narrowpocket P surrounded by the outlet of the nozzle and the sealing fins 5b. Consequently, this pocket P may be greatly scooped out by erosion orthe sealing fins 5 b may drop out. The possibility of this risk becomeshigh when geothermal steam includes a lot of solid particles, and thisis one of the main factors possibly adversely affecting the reliabilityof the geothermal turbine.

In order to avoid this disadvantage, some through holes 5 c are arrangedin a circumferential direction, connecting an exit of the stage with thepocket P surrounded by the nozzle exit and the sealing fins 5 b.However, when the number of the holes 5 b is small, the particles cannotbe discharged completely, and conversely, when the number is large, anamount of an associated steam bypassing the rotor blade 2 increases, andthereby decreasing the efficiency of the turbine, which may cause aproblem.

The problem of damage by the erosion or corrosion of a steam sealingportion of the geothermal turbine occurs not only at the blade tip, butalso in a steam sealing portion between the turbine rotor shaft 8 andthe nozzle diaphragm inner ring. Since the intensive rotational flow ofthe nozzle exit dominates also in this portion, the droplets and thesolid particles may damage fins 9 b arranged on a nozzle diaphragm side,and thereby the efficiency of the turbine often decreases.

Moreover, a labyrinth seal by arranging a high-and-low groove 8 a on therotor shaft 8, is adopted for raising sealing effectiveness, and that isthe same steam sealing structure of gland packing portion in which therotor shaft penetrates a turbine casing. However, the steam flowsthrough this labyrinth portion with high velocity including droplets,solid particles and corrosive components as mentioned above, and thusthe high-and-low groove 8 a will be shaved off over time.

In any event, maintaining long-term reliability or extension of a lifespan of the steam sealing performance between a rotating portion and astationary portion is one of the issues which should be solved for ageothermal turbine.

On the other hand, for the turbine inlet stages, relatively smallnozzles, having a throat width at the nozzle exit of 5 to 8 millimeters,are adopted. Usually, a strainer is installed in the inlet portion ofthe geothermal turbine, so as not to entrain large solid particles intothe turbine. On the other hand, depending on conditions of a well of thegeothermal site, such a strainer frequently becomes plugged up, andcleaning of the strainer is often required. Thus, unavoidably, course orlarge meshes, for example, two meshes per inch, may be used. These roughmeshes may cause a partial blockage of the nozzle throat due to solidparticles passed through the meshes, and thereby the output of theturbine may decreases significantly. And even if the nozzle throat isnot blocked, the throat becomes narrow due to scale deposits on thenozzle surfaces, and thus the output may decrease.

Furthermore, when corrosion or pitting corrosion occurs on a surface ofthe nozzle, the efficiency may be extensively affected. Especially,corrosive elements are active at steam conditions of inlet stages of thegeothermal turbine, and thus this portion is easily affected bycorrosion and/or pitting corrosion (erosion), whereby the surfaceroughness of both nozzle and blade may become severely deteriorated.Since the efficiency drop due to deteriorated surface roughness ofnozzle and blade is related to a relative value of the roughness againstblade size, the smaller the nozzle or blade is, the more extensive theinfluence becomes.

In addition, for a small nozzle, there is also a problem of damage onthe surface of the nozzle due to solid particles. Since the rate ofcurvature becomes large as for a smaller nozzle, the solid particlescannot follow the rapid turning of the stream because of inertia andcollide with nozzle surfaces. Consequently, a thin outlet portion of thenozzle profile may be damaged and the nozzle profile may be extensivelydeformed, whereby the efficiency may decline. When the damage is large,an accompanying exciting force against the rotor blades becomesexcessive, and the reliability of the rotor blades itself is alsoaffected.

Moreover, since geothermal resources are limited, partial load operationis occasionally carried out during nighttime hours, when electric powerdemand is lower. Such a style of operation is required especially whenthe underground resource of the steam tends to be exhausted due tolong-running service. At low load, the last stage blade operates farfrom its aerodynamic design point with a substantially reduced outputand pressure drop across the stage. This will cause the flow field to beunsteady with large back flow. This unsteady reversed flow spreadswidely as the load decreases, and the exciting forces on the bladebecome more intensive, resulting in large intensive vibrating stressesin the blade.

If the vibration stresses on a stress concentrated portion, such asblade connection, corrosion pits or erosion pits, exceed the fatiguestrength of materials which has been decreased under the corrosiveenvironment, there arises the possibility of cracking as the worst case.Thus, the reliability of geothermal turbines used for cycling loadoperation depends on how the vibration stresses of the last stage bladecould be suppressed applying effective damping structure.

SUMMARY OF THE INVENTION

In consideration of the above-mentioned situation, and one object ofthis invention is to provide a geothermal turbine having improvedresistance against corrosion and erosion of components, such as rotorblades, nozzles and turbine rotors, which are operated in severegeothermal steam environment, and having an extended life span byapplying effective countermeasures against age deterioration.

Another object of this invention is to provide a geothermal turbine thatprevents the steam sealing structure formed between a stationary portionsuch as a nozzle diaphragm and a rotating portion such as rotor bladesfrom mechanical damages due to impurities contained in the geothermalsteam, and effectively prevents breakage and damage of components of theturbine, to thereby keep stable turbine performance for a long term.

Another object of this invention is to provide a geothermal turbineexhibiting improved maintainability of turbine components operated undera severe geothermal environment, to thereby sustain the turbinecomponents and performance easily.

Other and further objects of this invention will become apparent upon anunderstanding of the illustrative embodiments to be described herein orwill be indicated in the appended claims while various other advantagesnot referred to herein will become apparent to one skilled in the artupon employment of the invention in practice.

According to one aspect of the present invention, there is provided ageothermal turbine, comprising, a turbine casing, a turbine rotor shaft,a plurality of nozzle diaphragm outer rings fixed to the turbine casing,a plurality of nozzle diaphragm inner rings located radially inside thenozzle diaphragm outer rings, a plurality of nozzles placed between thenozzle diaphragm outer rings and the nozzle diaphragm inner rings, whichform a steam passage, a plurality of rotor blades mounted on the turbinerotor shaft, which face the nozzles and form a blade cascade in acircumferential direction, a shroud arranged on tip of the rotor blade,an overhang attached to the nozzle diaphragm outer ring, which extendsdownstream of the nozzles and has an inner surface facing radiallyoutside the shroud, and a plurality of sealing fins which protruderadially outward from the shroud and face the overhang.

According to another aspect of the present invention, there is provideda geothermal turbine, comprising, a turbine casing, a turbine rotorshaft, a plurality of nozzle diaphragm outer rings fixed to the turbinecasing, a plurality of nozzle diaphragm inner rings located radiallyinside the nozzle diaphragm outer rings, a plurality of nozzles placedbetween the nozzle diaphragm outer ring and the nozzle diaphragm innerring, which form a steam passage, and a plurality of rotor bladesmounted on the turbine rotor shaft, which face the nozzles and form ablade cascade in a circumferential direction, wherein the nozzlediaphragm inner ring includes high-and-low sealing fins protruding froman inner surface of the nozzle diaphragm inner ring toward the turbinerotor shaft radially inward, and the turbine rotor shaft includes ahigh-and-low groove corresponding to the high-and-low arrangement of thesealing fins.

According to still another aspect of the present invention, there isprovided a geothermal turbine, comprising, a turbine casing, a turbinerotor shaft, a plurality of nozzle diaphragm outer rings fixed to theturbine casing, a plurality of nozzle diaphragm inner rings locatedradially inside the nozzle diaphragm outer rings, a plurality of nozzlesplaced between the nozzle diaphragm outer rings and the nozzle diaphragminner rings, which form a steam passage, a plurality of rotor bladesmounted on the turbine rotor shaft, which face the nozzles and form ablade cascade in a circumferential direction, and a strainer installedat an inlet of the turbine, wherein a throat is formed between adjacentnozzles, having a width larger than a mesh size of the strainer, and atleast a part of a surface of the nozzle is coated or overlay welded witha protective material.

According to still another aspect of the present invention, there isprovided a geothermal turbine, comprising, a turbine casing, a turbinerotor shaft, a plurality of nozzle diaphragm outer rings fixed to theturbine casing, a plurality of nozzle diaphragm inner rings locatedradially inside the nozzle diaphragm outer rings, a plurality of nozzlesplaced between the nozzle diaphragm outer rings and the nozzle diaphragminner rings, which form a steam passage, and a plurality of rotor bladesmounted on the turbine rotor shaft, which face the nozzles and form ablade cascade in a circumferential direction, wherein the rotor bladeincludes a plurality of cover pieces connecting tip portions of adjacentblades, each cover piece including a first tenon protruding from oneside of the cover piece and a first tenon hole on opposite side of thecover piece, the first tenon being inserted into a second tenon holearranged on a tip portion of a first blade, and a second tenonprotruding from a second blade next to the first blade being insertedinto the first tenon hole, whereby the blade is continuously coupled 360degrees around the wheel.

According to still another aspect of the present invention, there isprovided a geothermal turbine, comprising, a turbine casing, a turbinerotor shaft, a plurality of nozzle diaphragm outer rings fixed to theturbine casing, a plurality of nozzle diaphragm inner rings locatedradially inside the nozzle diaphragm outer rings, a plurality of nozzlesplaced between the nozzle diaphragm outer rings and the nozzle diaphragminner rings, which form a steam passage, and a plurality of rotor bladesmounted on the turbine rotor shaft, which face the nozzles and form ablade cascade in a circumferential direction, wherein each rotor bladeis coupled with a plurality of cover pieces connecting tips of adjacentblades, each cover piece including tenon holes on both sides of thecover pieces, a first tenon protruding from a first blade being insertedinto the tenon hole of the cover piece; a second tenon protruding from asecond side adjacent to the first blade being inserted into the tenonhole of the opposite side of the cover piece, whereby the blade iscontinuously coupled 360 degrees around the wheel.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description ofpreferred embodiments, when considered in connection with theaccompanying drawings wherein:

FIG. 1 is a sectional view showing a turbine stage of a geothermalturbine of a first embodiment in this invention;

FIG. 2 is a sectional view showing a steam sealing structure of a tip ofa rotor blade in a first modified example of the turbine stage shown inFIG. 1;

FIG. 3 is a sectional view showing a steam sealing structure of a tip ofa rotor blade in a second modified example of the turbine stage shown inFIG. 1;

FIG. 4 is a sectional view showing a turbine stage of a geothermalturbine of a second embodiment in this invention;

FIG. 5 is a sectional view showing a steam sealing structure of a rotorshaft portion of a turbine rotor of a geothermal turbine in thisinvention;

FIG. 6 is a schematic view showing nozzles of a geothermal turbine inthis invention;

FIG. 7 is a schematic diagram showing flow behavior around the nozzlesshown in FIG. 6;

FIG. 8 is a schematic diagram showing flow behavior on a trailing edgeside of the nozzles shown in FIG. 6;

FIG. 9 is a perspective view showing a connection structure for rotorblades of a geothermal turbine in this invention;

FIG. 10A and FIG. 10B are perspective views showing connection structurefor rotor blades of a geothermal turbine modified vis-à-vis the exampleshown in FIG. 9;

FIG. 11 is a sectional view showing a turbine stage of a typicalaxial-flow turbine applied to a conventional geothermal turbine.

FIG. 12 is a schematic view showing a connection structure of rotorblades of a conventional turbine; and

FIG. 13 is an exploded schematic view showing a connection structure ofrotor blades of a conventional turbine.

DETAILED DESCRIPTION OF PREFFERED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, preferredembodiments of a geothermal turbine in this invention will be describedbelow.

(First Embodiment)

FIG. 1 is a sectional view showing a turbine stage of an axial-flowturbine, typically applied to a geothermal turbine, of a firstembodiment in this invention. A geothermal turbine 10 has turbine stages12 concentrically arranged as structure of a plurality of stages in aturbine casing 11. Each turbine stage 12 is constituted by combining anozzle diaphragm outer ring 17, a nozzle diaphragm inner ring 18positioned inwardly in a radial direction of the nozzle diaphragm outerring 17, nozzles 19 arranged between the nozzle diaphragm outer ring 17and the nozzle diaphragm inner ring 18, and rotor blades 15 arrangeddownstream of and opposite to the nozzles 19. The nozzle diaphragm outerring 17 is contained and fixed in an inner peripheral slot 20 arrangedon the inner face of the turbine casing 11.

Each nozzle 19 constitutes a steam passage portion, and meanwhile, aplurality of sheets of nozzles 19 are arranged in a line in acircumferential direction between the nozzle diaphragm outer ring 17 andthe nozzle diaphragm inner ring 18, and form a nozzle cascade. Sealingfin attachment portions 21 are formed in the inner periphery of thenozzle diaphragm inner ring 18 and a plurality of sealing fins 22, inthe form of rings or washers, are attached to the sealing fin attachmentportions 21 appearing comb-like in section, constituting a packingportion.

A high-and-low groove 27 is arranged on an outer surface of the turbinerotor shaft 26, facing the sealing fins 22 of the nozzle diaphragm innerring 18. The high-and-low groove 27 is formed on the entirecircumference of the rotor shaft 26 and is composed of a plurality ofconvex portions 28 extending along a circumferential direction and aplurality of concave portions 29 arranged between the convex portions28. A minute gap 30 is formed between the sealing fins 22 on the nozzlediaphragm inner ring 18 and the high-and-low groove 27 of the turbinerotor shaft, and this minute gap constitutes a steam sealing portion 32of a labyrinth type. This steam sealing portion 32 enables the structureto prevent steam leakage along the turbine rotor shaft 26.

As for the steam sealing portion 32, a tip of each comb-like sealing fin22 is staggered, that is, a sealing fin 22 facing the concave portion 29of the high-and-low groove 27 protrudes toward the turbine rotor 26 sidelonger than a sealing fin 22 facing the convex portion 28. Herewith, thegap 30 formed between the sealing fin attachment portion 21 of thenozzle diaphragm inner ring 18 and the turbine rotor shaft 26 could beminute, and steam leakage can be effectively prevented by arranging theflow passage formed by the gap 30 to be staggered and long, to enlargepassage resistance of the steam sealing portion 32 of a labyrinth type.

Rotor blades 15 constituting the turbine stage 12 are arranged on theturbine rotor shaft opposed to, and downstream of, the nozzle portion14. Rotor blades 15 comprise a plurality of blades 36 mountedcircumferentially on a disk 35 of the turbine rotor shaft 26, and ashroud 37 is arranged at the tip of each of the blades 36. The shroud 37combines and unites several numbers of the blades 36, and thusconstitutes a group of blades. Above the blades 36, an overhang 38 islocated with extending downstream from the nozzle diaphragm outer ring17. The blades 36 constitute a steam passage, and meanwhile, rotateinside the overhang 38 of the nozzle diaphragm outer ring 17, whereby agap 40 is formed between the shroud 37 on the tip of the blades 36 andthe overhang 38 opposed to radially outside the shroud 37.

Some of the steam does not pass through the blades 36 and leaks throughthis gap without working. To prevent this leakage, a plurality ofsealing fins 41 are arranged as a comb-like in section on the outersurface of the shroud 37 along a circumferential direction extendingradially outward toward the overhang 38 of the nozzle diaphragm outerring 17. Thus, a steam sealing portion of the tip of the rotor blade isformed with keeping a gap 40 small between the tip of the sealing fins41 and the inner surface of the overhang 38 of the nozzle diaphragm 17.This steam sealing portion 44 forms a structure preventing steam leakageon the tip of the rotor blade.

Steam flow is shown in FIG. 1 as an arrow A. The nozzles 19 constitutinga steam passage are located upstream of the rotor blades 36 fordeflecting the steam flow in the circumferential direction and applyinga turning force to the blades 36. Thus, it is necessary to introduce thesteam discharged from the nozzles into the rotor blades 36 effectivelyin order to raise the efficiency of the geothermal turbine.

On the other hand, this geothermal steam includes water droplets andforeign substances, such as solid particles peculiar to the geothermalsteam, which are heavier in density than the steam. These droplets andforeign substances are moved peripherally to the exit of the nozzle 19by the effect of centrifugal force due to strong rotating flow of thesteam. Consequently, many droplets and foreign substances arecentrifuged just behind the nozzle exit to the periphery, that is,around a root portion 45 of the overhang 38 of the nozzle diaphragmouter ring 17.

Thus, in the geothermal turbine 10 shown in FIG. 1, sealing fins 41 arearranged on the periphery of the shroud 37 and opposed to the overhang38 of the nozzle diaphragm outer ring 17, without forming sealing finson the overhang 38 of the nozzle diaphragm outer ring 17. A plurality ofsealing fins, that is, for example, a few sealing fins, are arranged onthe outer periphery of the shrouds 37, appearing comb-like in section.

A small gap 40 is formed between the overhang 38 of the nozzle diaphragmouter ring 17 and the sealing fins 41 on the outer surface of theshrouds 37, and constitutes a steam sealing portion 44 at the tip pf therotor blade, and this steam sealing portion 44 forms structure forpreventing steam leakage on the tip portion of the rotor blade.Moreover, the inner surface 38 a of the overhang 38 of the nozzlediaphragm outer ring 17 is overlay welded or coated with a protectivematerial having a durability greater than the material of the overhang38, that is, a corrosion resistant material or an erosion resistantmaterial, for preventing corrosion due to corrosive components includedin the geothermal steam or erosion by droplets or solid particlesincluded in the geothermal steam.

A preferred erosion resistant material or erosion resistant material foroverlay welding is, for example, titanium, an alloy of cobalt (Co) as aprinciple ingredient, an alloy of chromium (Cr) or a mixture of chromiumand iron (Fe) or mixture of chromium, iron and nickel (Ni) as principleingredients, or an alloy of titanium (Ti) as a principle ingredient.Another preferred erosion resistant material for use as a coating is,for example, an alloy with cobalt as a principle ingredient, an alloy ofchromium (Cr) or mixture of chromium, iron and nickel as principleingredients, a composition of chromium such as chromium carbide, acomposition of titanium such as titanium nitride, or a composition oftungsten (W) such as tungsten carbide, is suitably used. A preferablecorrosion resistant material, or corrosion resistant material for use asoverlay welding is, for example, an alloy of chromium as a principleingredient, an alloy of nickel as a principle ingredient, an alloy ofmixture of chromium, nickel and iron as principle ingredients, an alloyof titanium as a principle ingredient, or an alloy of cobalt as aprinciple ingredient. As the corrosion resistant material for use as acoating, there is preferred, for example, an alloy of chromium as aprinciple ingredient, an alloy of nickel as a principle ingredient, analloy of a mixture of chromium, nickel and iron as principleingredients, an alloy of titanium as a principle ingredient, an alloy ofcobalt as a principle ingredient, or a composition of chromium, tungstenand titanium.

Moreover, overlay welding may be applied by welding such as plasmapowder welding or shielded metal arc welding, with a cermet made bymixing a carbide such as tungsten carbide (WC), chromium carbide (CrC),niobium carbide (NbC) or vanadium carbide (VC) or a boride such astitanium boride (TiB₂) into the above-mentioned corrosion resistantmaterials or the above-mentioned erosion resistant materials. Also,coating may be applied by plasma spraying such as atmospheric plasmaspraying (APS), or high velocity frame spraying such as high-velocityoxy-fuel (HVOF) spraying or high-pressure high-velocity oxy-fuel(HP-HVOF) spraying, with one of the above-mentioned metal substances orcermet as a spraying powder material. The particle diameter of thepowder of the erosion resistant materials, or the corrosion resistantmaterials, or the carbide or the boride forming the cermet, is desirably20 to 75 micrometers. If the particle diameter is larger than 75micrometers, the microstructure of the sprayed metal becomes rough, andthere may be some of particles which cannot be melted completely, andthis presents an issue of durability of the sprayed portion. On theother hand, particles, finer than 20 micrometers in diameter are notdesirable in terms of the difficulty of adjusting the spraying.

By arranging the steam sealing portion 44 in this construction, thewater droplets and foreign particles discharged at the root portion 45are pushed on the inner surface 38 a of the overhang 38 and rotate dueto the strong swirling flow of the steam, without staying at theoverhang 38 a because there are no obstacles damming the flow on theinner surface 38 a of the overhang 38, and thus, the water droplets andsolid particles flow along the smooth inner surface 38 a of the overhang38 and can be smoothly discharged into the exit of the turbine stage.They can then be captured by a drain catcher. Therefore, the rootportion 45 of the nozzle diaphragm outer ring 17 is not significantlydamaged due to erosion. If the overhang 38 is worn out, the gap betweenthe tip of the sealing fins 41 on the outer surface of the shroud 37 andthe overhang 38 of the nozzle diaphragm outer ring 17 would be enlarged,and this would cause a factor of age deterioration of the turbineefficiency; however, in this invention, the overlay welding or thecoating on the inner surface 38 a of the overhang 38 with theabove-mentioned protective material having a durability greater than thematerial of the overhang 38, that is, the corrosion resistant materialof the erosion resistant material is resistant against water droplets,corrosive components or solid particles, and aging wear-out of thegeothermal turbine can be prevented. Thus, the turbine efficiency can besustained at a high level over a long period of time.

In addition, an annular gap 40 is formed between the tip of the shroudfin 41 on the tip of the rotor blades 15 and the inner surface 38 a ofthe overhang 38 of the nozzle diaphragm outer ring 17; however, this gap40 is almost the same size as one in the conventional structure. And inthis embodiment, discharge holes for the foreign substance on theoverhang 38 of the nozzle diaphragm outer ring 17 is not necessary, andthus there is not an additional loss due to steam leaking through thedischarge holes to the turbine stage exit 47 as is the case in theconventional structure. As a result, the turbine efficiency can beraised based on this design.

Moreover, since water droplets thrown out to the inner surface of theoverhang 38 of the nozzle diaphragm outer ring 17 becomes a water coatcovering the inner surface 38 a of the overhang 38 by the effect ofcentrifugal force, this has the effect of narrowing the gap 40 to theextent of thickness of a water film, and thus the amount of steamleakage can be further decreased. Reduction of the steam leakagecontributes to the improvement of the efficiency of the turbine.

FIG. 2 shows a first modified example of the first embodiment of thestructure for preventing steam leakage at the tip of the rotor blade ofthe geothermal turbine. In the structure shown in FIG. 2, an overhang 50of the nozzle diaphragm outer ring 17 is formed as a cylindrical bodyapart from the body of the nozzle diaphragm outer ring 17, and thiscylindrical body is tied and integrated on the whole by attachment meanssuch as bolts. The overhang 50 of the nozzle diaphragm outer ring 17 isconstituted in the shape of a cylinder, a sleeve ring, or torus. Theoverhang 50 is manufactured with the corrosion resistant material or theerosion resistant material. By making the overhang 50 from substanceshaving excellent resistance against corrosion or erosion, the erosiondue to water droplets and solid particles or the corrosion due tocorrosive components on an inner surface 50 a of the overhang 50(constituting a steam sealing portion 44) can be effectively prevented.

If the overhang 50 is damaged due to operation of the geothermal turbine10 for a long period, the overhang 50 can be easily replaced, and thusthe ease of maintaining the turbine equipment can be raised. Since theoverhang 50 is made as a single body, there are many options ofmaterials for the overhang 50, and materials can be selected and changedas it is suitable for conditions of the geothermal steam at each site.Moreover, the advantages of the geothermal turbine 10 shown in FIG. 1can be attained. The other aspects of this embodiment are substantiallythe same as the one shown in FIG. 1, and therefore a detailedexplanation thereof is omitted.

FIG. 3 shows a second modified example of the first embodiment of thestructure for preventing steam leakage at the tip of the rotor blades ofthe geothermal turbine. In this modified example, an overhang 52 of thenozzle diaphragm outer ring 17 is formed as a cylindrical body apartfrom the body of the nozzle diaphragm outer ring 17, and a minute slit53 is arranged between the nozzle diaphragm outer ring 17 and theoverhang 52. This slit 53 has the function of discharging water dropletsand solid particles into the exit 47 of the turbine stage withoutpassing through the gap 40.

The slit formation portion 54 is arranged on the nozzle diaphragm outerring 17, and the outer surface of this slit formation portion 54 isoverlay welded or coated with the above-mentioned protective materialhaving a durability, that is, the corrosion resistant material or theerosion resistant material for preventing damage due to water dropletsor foreign substances. This overlay welding or coating may be partial,and it is not necessary to be formed over all the surface of the slitformation portion 54. As for the geothermal turbine, this slit 53 iseffective for protecting the sealing fins 41 of the steam sealingportion 44 and also the steam passage from damage in case there is ahigh concentration of foreign substances, such as solid particles, inthe geothermal steam. Similar to a case shown in FIG. 2, the overhang 52is manufactured, at least a part, from the above-mentioned protectivematerial, that is, the erosion resistant material or the corrosionresistant material, and the same advantages as of the geothermal turbine10 shown in FIG. 2 can be attained.

(Second Embodiment)

FIG. 4 is a sectional view showing a geothermal turbine of a secondembodiment in this invention. The geothermal turbine 10A shown in FIG. 4has a different structure for preventing steam leakage of the turbinerotor shaft 26 from the geothermal turbine 10 from that shown in FIG. 1.However, the other components of the geothermal turbine 10A are actuallythe same as those of the geothermal turbine 10, and a detailedexplanation of these components is omitted. The same reference numeralsare used as in FIG. 1.

In the structure for preventing steam leakage on the turbine rotor shaft26 shown in FIG. 4, sealing fin attachment portions 21 are arranged onan inner periphery of the nozzle diaphragm inner ring 18, and aplurality of sealing fins 22 are attached to these sealing finattachment portions 21, appearing comb-like in section in acircumferential direction. Moreover, a high-and-low groove 60 is formedaround the entire circumference of the surface of the turbine rotorshaft 26. The high-and-low groove 60 is constituted by a plurality ofconvex portions 61 extending along a circumferential direction and aplurality of concave portions 62 arranged between the convex portions61. Each of the convex portions 61 is formed trapezoidal in section, asshown in FIG. 4, so that it tapers odd radially outside at the tip. Byarranging the convex portion 61 trapezoidal in section, each of theconcave portions 62 is also made trapezoidal or dished in section.Moreover, it is also possible that this high-and-low groove 60 is formedtrapezoidal or semicircular or some other part-circular orpart-elliptical or arched section.

A gap 30 is formed between the sealing fins 22 arranged on the innercircumferential side of the nozzle diaphragm inner ring 18 and thehigh-and-low groove 60 on the outer surface of the turbine rotor shaft26, and this gap 30 constitutes a steam sealing portion 63 of labyrinthstructure. This steam sealing portion 63 forms a structure forpreventing steam leakage on the turbine rotor shaft 26, whicheffectively prevents the steam leakage passing through the gap 30. Atleast one surface of the high-and-low groove 60 of the steam sealingportion 63 is covered with the above-mentioned protective material, thatis, the corrosion resistant material or erosion resistant material. Byarranging the high-and-low groove 60 trapezoidal in section, thecollision angle of the water droplets or solid particles, entrained bythe high-speed steam passing through the gap 30 of the steam sealingportion 63, toward a side face of the trapezoid of the groove 60 isminimized. Thus, the collision energy that the high-and-low groove 60receives is decreased compared to a conventional case of a perpendicularcollision, and abrasion of the high-and-low groove 60 is also reduced.Moreover, by arranging the high-and-low groove 60 trapezoidal insection, the spraying application of a coating material onto all thesurfaces of the high-and-low groove 60, including side faces of theconvex portion, can be effectively, firmly and smoothly performed.

A general method for coating on the surface of metals is to spray heatedparticles of the coating material onto the metal surface with highspeed, and it is necessary to keep the spray angle onto the metalsurface around 90° to ensure good adhesion. The shape of a conventionalhigh-and-low groove cannot satisfy this condition and cannot performfine coating because the side face of the convex portion 28 isperpendicular to the rotor shaft as shown in FIG. 11. However, thehigh-and-low groove 60 of the rotor shaft 26 shown in FIG. 4 has atrapezoidal shaped in section, and thus in this embodiment, fine coatingcan be realized.

Leakage steam passing through this gap 30 bypasses the nozzles 19 andflows through the steam sealing portion 63; however, this leakage steamdoes not contribute any work of the turbine stage 12 and may become afactor decreasing the turbine efficiency. Thus, by adopting a labyrinthstructure for the steam sealing portion 63, the steam leakage can beeffectively prevented.

In addition, this structure for preventing steam leakage can be appliednot only between the rotor shaft 26 and the nozzle diaphragm inner ring18 but also a gland packing portion in which the turbine rotor shaft 26penetrates the turbine casing 11. Moreover, FIG. 4 shows an example ofthe structure for preventing steam leakage applied to the turbine rotorshaft 26 of the geothermal turbine 10 shown in FIG. 1; however, thisstructure for preventing steam leakage can be also combined with thestructure for preventing steam leakage of the blade tip portion shown inFIG. 2 or FIG. 3.

FIG. 5 shows a modified example of the geothermal turbine of thisembodiment in the invention. This modified example is different from thestructure for preventing steam leakage of the turbine rotor shaft shownin FIG. 4, but the other structure of this example is substantially thesame as that in FIG. 4, and therefore a detailed explanation thereof isomitted. In the structure for preventing steam leakage of the turbinerotor shaft 26 shown in FIG. 5, a detachable sealing fin segment 66 iscomposed of a separate body from the nozzle diaphragm inner ring 18 thatis connected to the nozzle diaphragm inner ring 18 and integrated on thewhole by attachment means such as bolts, and this sealing fin segmentcan be both attached and detached. The detachable sealing fin segment 66equipped with a mounting flange 66 a is formed in the shape of acylinder, a torus or a sleeve ring, manufactured with the erosionresistant material or corrosion resistant material.

The structure of this modified example, except for the detachablesealing fin segment 66 as a separate piece on an inner periphery of thenozzle diaphragm inner ring 18, is not substantially different from thestructure for preventing steam leakage of the turbine rotor shown inFIG. 4. In the geothermal turbine 10A, water droplets or solid particlesare contained in the steam passing through the steam sealing portion 63.In the conventional structure for preventing steam leakage shown in FIG.11, water droplets and solid particles contained in the steam passingthrough a gap of the steam sealing portion collide with the convexportion of the high-and-low groove 8 a of the rotor shaft 8 andafterwards scatter by the effect of centrifugal force and collide withthe inner surface 9 a of the nozzle diaphragm inner ring 9. Thus, theinner periphery of the nozzle diaphragm inner ring 9 is damaged. If aroot of the sealing fin 9 b is implanted In an inner periphery of thenozzle diaphragm inner ring 9, is damaged, the sealing fin 9 b dropsout, and thus the efficiency of the turbine falls significantly.

However, in the structure for preventing steam leakage of the turbinerotor 25 shown in FIG. 5, the detachable sealing fin segment 66 isattached on an inner periphery of the nozzle diaphragm inner ring 18,and the detachable fin segment 66 is removable and made, at leastpartly, from the protective material, that is, the material havingexcellent resistance against erosion and/or corrosion attack. Thus, thedetachable sealing fin segment 66, with excellent materialcharacteristics, prevents the inner surface 66 b, on which the sealingfin 22 is attached, from being exposed by erosion due to water dropletsor solid particles or corrosion due to corrosive components.

Even if the steam sealing portion receives damage due to long-termoperation of the geothermal turbine 10A, the sealing fins can be easilyreplaced by detaching this detachable sealing fin segment 66, and thusthe maintainability of the turbine equipment is improved. Since thedetachable sealing fin segment 66 is made as a single body, there aremany options for the materials of the sealing fin segment 66, andcombined with the rotor shaft 26 structure shown in FIG. 4, thereliability of the steam sealing portion 63 of the turbine rotor shaftcan be maintained for a long time.

In the structure of preventing steam leakage of the rotor shaft 26 ofthe turbine rotor 25 shown in FIGS. 4 and 5, the high-and-low groove 60is shown in the exemplary form of trapezoidal section in the axialdirection on the outer surface of the rotor shaft 26; however, the crosssection of the high-and-low groove 60 may have other shapes, e.g., aconvex portion and/or a concave portion of the form of arc, half-circle,semicircle, half-oval, semi-oval, half-ellipse, or semi-ellipse.

In the geothermal turbine 10, the turbine inlet stages, namely, thefirst, second and third turbine stages are forced to operate underespecially severe operating conditions. That is, firstly, they areaffected by corrosive components contained in the geothermal steam.Corrosive components contained in the geothermal steam are active in thetemperature range of between 180 to 200° C., and the operatingtemperature of the steam at one of the turbine inlet stages of thegeothermal turbine 10 is present at this active temperature range duringexpansion, and thus the relevant range is under a very corrosivecondition. Secondly, these stages are affected by solid particles.Generally, a strainer (not illustrated) is installed at the inlet of theturbine in order to prevent foreign substances, such as the solidparticles, from entering the geothermal turbine 10, and ordinarily thestrainer has four or five meshes per inch, so that relatively largesolid particles never come into the geothermal turbine.

However, if the condition of a geothermal well is not good and a highconcentration of solid particles is included in the steam, theabove-mentioned finer mesh becomes clogged up soon, and thus, a courserstrainer having only two meshes per inch is must be used. Largeparticles passed through this strainer having a large mesh directly hitthe nozzles of the turbine inlet stages, and may cause damage due to thecollision and/or erosion. In the turbine inlet stages, the more severecorrosive environment worsens the damages. Moreover, with the use of twomeshes per inch, there is the possibility that relatively largeparticles having a diameter up to 12 millimeters pass through and thusthe conventional nozzles having smaller throat width of 5 to 8millimeters may be blocked up.

In the geothermal turbine of one embodiment of this invention, nozzles70 used in the turbine inlet stages are arranged as shown in FIG. 6. Thechord length of each nozzle 70 constituting a nozzle cascade 71 is atleast twice as large as a conventional one, and the throat width at thenozzle exit formed between adjacent nozzles is set as ½ inch or more.When each nozzle forming nozzle cascade is designed as shown in FIG. 6and the throat width at the nozzle exit is arranged to be ½ inch ormore, consequently, the throat width 73 of the nozzle 70 is equal to ormore than the mesh size of the strainer, the nozzle cannot be blocked upeven with the largest particles passing through the mesh of thestrainer.

When nozzles 70 having a relatively large pitch and a relatively largechord length are adopted for the nozzle cascade 71, the number of thenozzles becomes relatively small, and it gets difficult for the lowermodes of the natural frequencies of the rotor blades to avoid resonancewith NPF (a product of the number of nozzles and the rotationalfrequency of the rotor shaft). However, for example, excellent dampingcharacteristics against vibratory excitation can be obtained bycombining the nozzles 70 with snubber blades, which have integralshrouds on the tip of the rotor blades and the side faces of the shroudare being pressure contacted to the adjacent ones each other, and thusthe blades being continuously coupled 360 degrees, therefore, thecontact between adjacent shrouds limits the amplitude of vibration andproduces high damping when stimulating steam forces work.

FIG. 7 is a drawing for explaining steam flow and the behavior of solidparticles passing through the nozzle cascade. The geothermal steamguided into the geothermal turbine 10 flows around the nozzles 70 asshown by dashed line B. On the other hand, water droplets and solidparticles contained in the steam flow as shown in solid line C due tothe large density. They intensively collide with the leading edge 70 athrough the concave side 70 b of the nozzle, giving erosion damage inthis area. On the other hand, the boundary layer flow on the convex sideof the nozzle gradually progresses and becomes thicker at the trailingedge. In a lower layer portion of the boundary layer close to the wall,steam velocity is quite low due to viscosity of the fluid, and as aresult, that portion is always covered with condensed water containingcorrosive elements. In the upper layer part of the boundary layer,however, flow velocity is as fast as the main flow, and thus a vortexflow is generated due to the interaction of such low and fast flows.This vortex flow causes interaction of the corrosive components andsolid particles, and thus, the trailing edge of the convex side isexposed to a condition that can easily cause damage due to erosionand/or corrosion.

This embodiment is focused on this point. In the inlet stages of thegeothermal turbine 10 shown in FIG. 6, all of the surface of the nozzle70, or at least a part of the surface from the leading edge 70 a to theconcave side 70 b of the nozzle 70, which can be easily damaged due toerosion based on collision of the droplets or the solid particles, andthe trailing edge portion 70 c of the convex side, which can be easilydamaged by corrosion due to the corrosive components retained in thewater film of the boundary layer 76, is coated with a material havingexcellent corrosion resistance and/or erosion resistance. By coating atleast necessary part of the surface of the nozzle 70 with the materialhaving excellent corrosion resistance and/or erosion resistance, thenozzle has the function of protecting the surface from attacks of boththe solid particles and the corrosive components and maintaining theshape of the nozzle for a long time.

As for the nozzle 70 shown in FIG. 6 having a size larger than twice thesize of the conventional one, even if scales are deposited on thesurface of the nozzle, the reduction of the throat width 73 due to thescale is minute, and reduction in swallowing capacity is also minute.Thus, the decrease of output over time due to the scale deposit on thesteam passage can be minimized, and stable operation can be maintainedfor a long time. Failures in surface roughness and profile shape of thenozzles due to erosion and/or corrosion damages, observed in aconventional technique, cause not only deterioration of reliability butalso significant degradation of the efficiency of the turbine.Accordingly, the coating of the surface of the nozzle 70 as shown inFIGS. 6 through 8, for preventing degradation of the efficiency,contributes heavily to protection for energy resources.

(Third Embodiment)

FIG. 9 shows a third embodiment of a geothermal turbine according tothis invention. FIG. 9 shows connection structure of a tip of a rotorblade, which is suitable for the turbine latter stages having relativelylong blades. In this connection structure of the tip portion of therotor blade, tip portions 81, 81 of the rotor blades 80 a, 80 b, whichare adjacent each other, are connected with a cover piece 82. This coverpiece 82 has a block configuration such as a rhombus or a parallelogram.A tenon 83 protrudes on one side of the cover piece 82, and a tenon hole84 is formed on a counter side of the cover piece 82 opposite to theside from which tenon 83 protrudes. The tenon 83 protruded from one sideof the cover piece 82 is inserted into a tenon hole 85 bored in a tipportion of the rotor blade 80 a and is fixed by deforming the top of thetenon 83. Alternatively, the tenon 83 may be fixed by inserting tightlyinto the tenon hole 85 of the tip portion of the rotor blade 80 a.

In case of the deforming of the top of the tenon 83, the deformed top ofthe tenon completely covers and closes the periphery of the tenon hole85, and the side of the cover piece 82 adhered closely to a flat portionof an inlet side 86 of the tenon hole 85 closes the tenon hole 85, whichprevents the corrosive components from entering into an inner face ofthe tenon hole 85. In the case of tight insertion of the tenon 83 intothe tenon hole 85, the side of the cover piece 82 also adheres closelyto the flat portion of an inlet side 86 of the tenon hole 85, whichprevents the corrosive components from entering into an inner face ofthe tenon hole 85. A tenon 87 formed on a tip portion 81 of the rotorblade 80 b is loosely inserted into a tenon hole 84 formed on a surfaceopposite to a tenon 83 side of the cover piece 82.

In this constitution, by sequentially connecting adjacent blades 80 a,80 b, . . . , of the rotor blades 80 through the cover pieces 82, all ofthe blades 80 a, 80 b, . . . , mounted along the circumferentialdirection of the turbine rotor shaft are loosely connected to eachother, and thus the rotor blades 80 constitute, so called continuousloose coupling 360 degrees around the wheel. This structure of thecontinuous loose coupling around the wheel provides excellent dampingcharacteristics of the rotor blade against vibration. By applyingmaterials having excellent corrosion resistance and erosion resistanceto the cover pieces 82, corrosion inside the tenon hole 84, which isloosely assembled, and erosion due to droplets can be prevented. Thisconnection structure achieves advantages in comparison to theconventional connection structures shown in FIGS. 12 to 13 operating ina corrosive environment, and enables stable cycling load operation ofthe geothermal turbine for a long time.

FIGS. 10A and 10B show a modified example of the connection structure ofthe rotor blade according to this embodiment. FIG. 10A is a perspectiveview, and FIG. 10B is a drawing viewed from the top of the blade. Thisconnection structure of rotor blades shown in this modified example isalso composed of blades 80 a, 80 b, which are adjacent each other, ofthe rotor blades 80, and a cover piece 90 connecting tip portions 81 ofthe blades 80 a, 80 b. The cover piece 90 is formed in a blockfiguration, such as a rhombus or a parallelogram, and tenon holes 91, 92are formed on both sides of the cover piece 90 facing each other.

On the other hand, tenons 93, 94 protrude on the sides facing each otherof the tip portion 81 of each of the blades 80 a, 80 b of the rotorblades 80, and these tenons 93, 94 are loosely inserted into the tenonholes 91, 92 of the cover piece 90, respectively. As explained above, bysequentially connecting tip portions 81 of blades 80 a, 80 b, . . . , ofthe rotor blades 80 through the cover pieces 82, all of the blades 80 a,80 b, . . . , of the rotor blades 80 in the circumferential directionare loosely connected to each other, and thus the rotor blade 80constitutes, so called continuous loose coupling 360 degrees around thewheel. This structure of the continuous loose coupling around the wheelprovides excellent damping characteristics of the rotor blade againstvibration. By applying the above-mentioned protective material, havingexcellent corrosion resistance and erosion resistance, to the coverpieces 90, corrosion inside the tenon holes 91, 92, which are looselyassembled, and erosion due to droplets can be prevented. Similar to theconnection structure shown in FIG. 9, this connection structure achievesadvantages in corrosive environment, and enables stable cycling loadoperation of the geothermal turbine for a long time.

In addition, in FIGS. 9, 10A and 10B, “loosely” designates a situationin which movement according to, for example, vibration of adjacentblades is not restricted. The tenon hole contacts with the tenon to someextent, and the movement is attenuated by friction of the tenon and thetenon hole. On the other hand, “tightly” designates a situation in whichthe tenon inserted into the tenon hole cannot be extracted by forcesequivalent to vibration of the blade, i.e., the blade vibrates togetherwith the cover piece as one body, and a relatively intensive force isnecessary to extract the tenon from the tenon hole.

Although the geothermal turbine of this invention is operated undersevere conditions in which steam includes corrosive components, waterdroplets and solid particles, this invention improves corrosionresistance and erosion resistance of the turbine structural components,such as turbine rotor shaft, nozzles, rotor blades, and steam sealingstructure between stationary parts and rotating parts, significantlydecreases aging deterioration of such components, and enables highefficiency operation and extends the lives of the turbine.

Moreover, the structure for preventing steam leakage formed betweenstationary parts, such as the nozzles, and rotating parts, such as therotor blades, significantly decreases aging deterioration brought aboutby foreign substances in the geothermal steam, effectively preventsdamage, problems and the degradation in the efficiency of the turbine,enables stable operation of the geothermal turbine for a long time andpromotes more effective use of energy resources for stable electricpower supplies.

Furthermore, maintainability of turbine components of the geothermalturbine operated under severe conditions can be improved, and thus,occasions of shut down for maintenance, inspection and repair of theturbine components and emergency shut down due to troubles aredecreased, and the operational availability of the geothermal turbineplant can thereby be raised. And since reliability at cycling loadoperation of the geothermal turbine improves, whereby operationalflexibility of the turbine plant increases, the plant can be operatedefficiently. Thus, saving and effective use of geothermal resources canbe realized.

The foregoing discussion discloses and describes merely a number ofexemplary embodiments of the present invention. As will be understood bythose skilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting, of the scopeof the invention, which is set forth in the following claims. Thus, thepresent invention may be embodied in various ways within the scope ofthe spirit of the invention.

1. A geothermal turbine, comprising: a turbine casing; a turbine rotorshaft; a plurality of nozzle diaphragm outer rings fixed to the turbinecasing; a plurality of nozzle diaphragm inner rings located radiallyinside the nozzle diaphragm outer rings; a plurality of nozzles placedbetween the nozzle diaphragm outer rings and the nozzle diaphragm innerrings, which form a steam passage; and a plurality of rotor bladesmounted on the turbine rotor shaft, which face the nozzles and form ablade cascade in a circumferential direction; wherein the rotor bladeincludes a plurality of cover pieces connecting tip portions of adjacentblades, each cover piece including a first tenon protruding from oneside of the cover piece and a first tenon hole on opposite side of thecover piece, the first tenon being inserted into a second tenon holearranged on a tip portion of a first blade, and a second tenonprotruding from a second blade next to the first blade being insertedinto the first tenon hole, whereby the blade is continuously coupled 360degrees around the wheel.
 2. The geothermal turbine as in claim 1wherein the cover pieces comprise a protective material having adurability greater than the material of the rotor blade.
 3. A geothermalturbine, comprising: a turbine casing; a turbine rotor shaft; aplurality of nozzle diaphragm outer rings fixed to the turbine casing; aplurality of nozzle diaphragm inner rings located radially inside thenozzle diaphragm outer rings; a plurality of nozzles placed between thenozzle diaphragm outer rings and the nozzle diaphragm inner rings, whichform a steam passage; and a plurality of rotor blades mounted on theturbine rotor shaft, which face the nozzles and form a blade cascade ina circumferential direction; wherein each rotor blade is coupled with aplurality of cover pieces connecting tips of adjacent blades, each coverpiece including tenon holes on both sides of the cover pieces, a firsttenon protruding from a first blade being inserted into the tenon holeof the cover piece; a second tenon protruding from a second sideadjacent to the first blade being inserted into the tenon hole of theopposite side of the cover piece, whereby the blade is continuouslycoupled 360 degrees around the wheel.
 4. The geothermal turbine as inclaim 3, wherein the cover pieces comprise a protective material havinga durability greater than the material of the rotor blade.